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Man-Made Vitreous Fibers (MMVF)

Introduction :

Man-made vitreous fibers (MMVF) or
synthetic vitreous fibers (SVFs) are a class of insulating
materials used widely in residential and industrial settings; they
are made primarily from glass, rock, slag or clay. The three
general categories are fiberglass, mineral wool , and refractory
ceramic fibers.

In some situations, SVF materials can
release fine, airborne dust particles, some of which are small
enough to be respirable. Thus, workers may be exposed to SVF
fibers by dermal contact and/or by inhalation.

Fibrous particles having long, thin
geometry present a special problem to the resiratory tract;
because fibers are thin, they can penetrate into the deep lung
and, because they are long, mobile lung cells may have difficulty
in removing them.

The three general categories may
be divided in the following manner :

A-Fiber Glass :

1-Glass Wool,

2-Continuous Filament

B-Mineral Wool :

1-Rock Wool,

2-Slag Wool.

C-Refactory Ceramic Fibers
:

1-Pure Oxides

2-Kaolin.

SVFs can help to control heat flow,
absorb acoustic energy, filter impurities from gases and liquids,
and with a vapor barrier, control condensation.

An important attribute of these fibers
is that they do not split longitudinally as do asbestos fibers.
Because asbestos fibers tend to split longitudinally, over time in
the lung, the number of lung, thin asbestos fibers can actually
increase, resulting in increasing lung irritation even after
exposure to asbestos has stopped.

In contrast, SVFs tend to break
tranversely into shorter segments, which the lung can clear more
readily than it can long fibers.

Synthetic Vitreous Fibers
:

A-Fiberglass :

Fiberglass is produced in two basic
forms, wool fibers and textile fibers.

1-Glasswool fibers :

The current major uses of glass wool are
in commercial and residential thermal insulation, noise-control
(acoustic) products, linings for air-handling ducts, pipe
insulation, air filters, roof insulation, and insulation for
automobiles, aircraft, mobile homes, refrigerators, domestic
cooking appliances, and a wide variety of other appliances and
equipment.

Submicron glass wool fibers, also known
as glass microfibers , are generally used in high-technology
products, such as high-efficiency particulate air filters,
specialty filter papers, battery components, and aerospace
insulation.

2-Glass textile fibers :

Also called continuous filament, they
are used in curtains and draperies, screening, electrical yarns,
roofing paper, shingles, and industrial fabrics and as
reinforcement for plastics, papers, rubber, and other
materials.

B-Mineral Wool :

Mineral wools include rock or stone wool
and slag wool. After formation, the materials are sprayed with
lubricating oils and binders to reduce dustiness (mineral wools
generally contain a very high ratio of nonfibrous particles, or
shot) and fiber breakage.

Mineral wool applications are very
similar to those of glass wool-thermal insulation, including fire
protection, and acoustic insulation. Much of the mineral wool
produced is used for blown-in insulation in attics and side walls.
Another popular use of mineral wool is in the manufacture of
decorative and acoustic ceiling tiles for commercial
building.

C-Refractory Ceramic Fiber
:

Refractory ceramic fiber (RCF) is
formulated to help control heat flow in high-temperature,
industrial situations. All RCFs are blends of alumina and silica
and other refractory oxides.

The three general categories of RCFs
are :

1-Kaolin clay based products, for which
the clay is obtained by mining.

3-High-purity products that are a blend
of purified alumina and silica and other materials.

Applications vary for RCFs, but all are
used in high-temperature, industrial environments.

RCF blankets are used as furnace and
kiln liners, as backup insulation to refractory brick, as soaking
pit covers, and in annealing welds. Loose RCF is used as a filler
in packing voids and in expansion joints. Custom-molded shapes of
RCF are used widely in metal molding, in catalytic converters, and
as combustion chamber liners in industrial furnaces.

Health effects :

A-Skin Irritation :

SVFs may irritate the skin of some
workers who are engaged in manufacturing, fabricating, or
installing SVF products. This irritation is a mechanical reaction
to sharp, broken ends of fibers that rub or become embedded in the
outer layer of the skin and does not appear to be an allergic
response. Typically, irritation does not persist and can be
relieved by washing exposed skin gently with warm water and mild
soap.

B-Upper Respiratory Tract Irritation
:

If large amounts of airborne fine fiber
are released during manufacturing or handling of SVF products, and
improper work practices permit inhalation of the fibers, some
workers may experience temporary upper respiratory
irritation.

The irritation consists of a
nonspecific, temporary respiratory condition, usually manifested
by coughing or wheezing. It is mechanically induced by sharp
fibers and does not appear to be an allergic reaction. It subsides
soon after the worker is removed from exposure and should have no
further impact on his or her health and well-being.

C-Safety Precautions :

Occupational health professionals
recommend three levels of precautions for protecting people when
they are manufacturing or handling SVF materials.

1-Whenever possible, SVF products should
be engineered and designed to limit their release of airborne
dust.

2-Manufacturing processes and controls
should be used to minimize airborne dust in the work
environment.

3-People should wear approved
respiratory protection and clothing that covers the skin as much
as possible when handling or installing SVFs.

Two major mortality studies have been
conducted on large groups of workers engaged in the production of
either glass or mineral wool, one in Europe and one in the
USA.

A third one, more limited, was conducted
in Canada on fiberglass production workers.

1-European study :

The researchers found an overall
mortality excess among the SVF workers, with the excess
particularly evident among workers with less than 1 year of
employment.

Among the causes of death that were more
numerous were malignant neoplasms, mental disorders,
cardiovascular diseases, respiratory diseases, digestive diseases,
and external causes.

a-Rock wool-slag wool workers :

Simonato et al. reported in 1987 an "
excess of lung cancer among rock wool-slag wool workers employed
during an early technological phase before the introduction of
dust-suppressing agents ", and concluded that " fiber exposure,
either alone or in

combination with other exposures, may
have contributed to the elevated risk ".

In their latest update, they concluded
that "the ensemble of these results is not sufficient to conclude
that the increased lung cancer risk is related specifically to
MMVF( SVF); however, insofar as respirable fibres were a
significant component of the ambient pollution of the working
environment, they may have contributed to the increased risk
"

b-Glass wool workers :

For these workers the report stated that
the findings " indicate some excess of lung cancer , clearly
reduced once local adjustment factors are applied to national
mortality rates, and with no relation to duration of employment
nor time since first employment ".

No anomalies were found for the
continuous glass filament workers.

c-Mesothelioma :

Five mesotheliomas have been identified
by death certificate, one in the glass wool sub-cohort and four in
the rock wool-slag wool sub-cohort. No clear increased risk of
mesothelioma has been identified, altough the researchers have
concluded that

" the possibility of such increase is
suggested by the results. "

2-American study :

As with the European study, the U.S.
study found a higher overall mortality rate among SVF workers as
compared to local and national mortality.

For deaths due to cancer or nonmalignant
respiratory disease, the study reported that a positive evidence
existed for fine glass and mineral wool production workers.

However, the researchers point out, the
data are not consistent with a causal relationship because the
excesses in mineral wool and glass microfiber deaths were not
directly related to duration of exposure.

The number of deaths from mesothelioma
in the study cohort is considered to be within the expected range
for the general population.

In the 1985 update, a small but
statistically significant excess in respiratory cancer deaths was
reported for workers employed in glass wool and mineral wool
plants but, the researchers concluded that the evidence of an
association appeared " somewhat weaker " than in the 1982
update.

In the 1989 update for the rock wool and
slag wool workers, the pattern of findings was generally
consistent with findings obseved in previous updates; no
consistent evidence remains of an association between lung cancer
or non-malignant respiratory diseases and any of the respirable
fiber measures considered.

3- Canadian study :

It is a more limited mortality study.
The authors reported a statistically significant excess in
mortality due to lung cancer among fiberglass production workers.
They concluded that the interpretation of this finding was
difficult because no relationship existed berween the excess of
lung cancer and the lenght of time since first exposure to the
fiberglass production environment.

It is concluded that the relationship
between work and health in the SVF industry should continue to be
explored.

B-Fiberglass and Mineral Wool
Morbidity Studies :

In the most widely cited SVF morbidity
study, Weil et al.(1983-1984) reported that the study populations
were generally healthy, with no respiratory symptoms and no
adverse lung functions related to the fiber exposure.

A low incidence of small lung opacities
was observed in the chest radiographs (opaque areas sometimes
observed in the lungs of workers in potentially dusty
trades).

In summarizing their findings they noted
that, in general, " the minimal evidence of respiratory effects
detected in the investigation , which cannot, at present, be
considered clinically significant, is encouraging concerning the
question of potential health effects of exposure to MMFV ".

This study was updated and enlarged at
the end of the 1980s and the authors concluded that the " results
indicate no adverse clinical, functional, or radiographic signs of
effects of exposure to MMVFs in these workers ".

C-Refractory Ceramic Fiber Morbidity
Studies :

Only one known published report is found
in the medical literature on health effects of occupational
exposure to RCFs. The researchers reported an association between
exposures to RCF and the occurrence of pleural plaques, which are
usually caused by exposure to asbestos fibers. It was demonstrated
that asbestos fibers exposure did not account for the observed
association.

Also, among the RCF workers, no
significant increase was seen in parenchymal changes consistent
with interstitial fibrosis.

Animal Toxicological
Studies :

A-Animal Implantation Studies
:

Implantation studies artificially inject
fibers into the body cavities of laboratory animals : into the
pleural (chest) cavity or peritoneal (abdominal) cavity or by
instillation into the trachea.

Implantation experiments are based on
introducing large amounts of fiber into animals by artificial
means that bypass normal body defenses. The circumstances of
actual exposure are totally different in humans. For these
reasons, and because the toxicology induced by implantation of
fibers into rodents does not parallel the findings from inhalation
studies, implantation studies are not valid for risk assessment or
for concluding anything about the human health hazard associated
with the inhalation of airborne SVFs.

On the other hand, implantation studies
have provided useful information on the mechanisms of fiber
toxicity. For example, long fibers (longer than 10 to 20 µm)
are most active in implantation as well as in cell culture
studies, so scientists have hypothesized that biological activity
is directly associated with fiber length.

B-Animal Inhalation Studies
:

The animal inhalation model is currently
the only valid laboratory method for assessing the hazard to
humans of exposure to airborne SVFs.

In recent chronic studies, test SVFs
having similar dimensions but different compositions have induced
different biological effects. Biological effects approximately
parallel fiber biological persistence in the lung. Fiber
compositions that are more lung-persistent would accumulate during
a chronic exposure and persist longer after termination of
exposure and would, therefore, cause more lung irritation than
compositions that dissolve or fragment transversely into shorter
segments.

Differences in biological effects could
also be related to fiber surface reactivity.

1-Fiberglass :

In the 1970s and 1980s, seven different
rodent inhalational studies reported no tumorigenesis for several
forms of fiberglass. In a recent study in rats fiberglass did not
induce fibrosis or tumors, whereas crocidolite asbestos induced
both types of lung disease.

In an other study (preliminary results)
in hamsters conducted recently comparing amosite fibers, 901
insulation wool and durable 475 glass demonstrated, as in other
studies, that no permanent lung changes were caused by 901 wool.
475 fiberglass induced minimal lung fibrosis and one tumor, a
mesothelioma. Amosite asbestos also induced fibrosis, but an
earlier and more severe case than that induced by 475 glass, and a
low to moderate incidence of mesothelioma. Differences between
this study and earlier ones, as far as glass wools are concerned,
seem to be related to differences in experimental
conditions.

In this study, toxicity somewhat
parallels lung biological persistence of fibers. After 12 months
of exposure, the number per lung of fibers longer than 20 µm
was seven times to eight times higher for high-dose amosite than
for 475 glass, which was three to four times higher than for 901
glass.

In agreement with previous findings,
these data once again link fiber lenght and biological persistence
to toxicity.

2-Mineral Wool :

Before 1990 three inhalation studies
reported no fibrosis or tumors as a result of chronic exposure to
mineral wool. One more recent study showed that rats exposed to
rock wool developed minimal fibrosis late in the inhalation
period.

3-Refractory Ceramic Fibers
:

Two inhalation studies of RCFs were
published before 1990, with conflicting results.

The second study (Smith et al.,1987)
reported RCF-associated fibrosis but no tumors in rats; no
fibrosis and only one mesothelioma in hamsters .

In more recent studies rats exposed to
RCF developed lung fibrosis, pulmonary tumors (13% in the
kaoalin-based RCF group) and pleural mesothelioma. Hamsters
exposed to RCF(exposed only to kaolin-based RCF) developed lung
fibrosis but no lung cancers but, 42 of 112 animals developed
mesotheliomas.

This study presents a striking
difference between rat and hamster responses to the same test
fiber that opens questions of species-related differences and
which species, if either, is representative of humans.

C-Cell Culture Studies :

A number of in vitro studies have shown
that fiber toxicity to cultured cells is related directly to fiber
lenght and perhaps indirectly to fiber diameter. In vitro studies
have also contributed much to a better understanding of the
molecular mechanisms of fiber-induced injury.

Fibers induce an inflammatory response
on the part of the lung and the activated inflammatory cells, in
an attempt to destroy foreign invaders, release biologically
destructive agents that also injure lung tissue. Repair and cell
proliferative responses to injury ensue. If the initiating fibers
are biologically persistent, the cascade continues and expands and
could result in increasing lung injury, repair mechanisms, and
possibly, permanent lung damage such as fibrosis or even
tumorigenesis.

Fiber biological
persistence & biotransformation :

A-In Vivo Studies :

Biological persistence of fibers is the
ability of fibers to persist in the lung after they have been
inhaled.

Biotransformation is any change in
dimension, composition, or surface morphology that occurs in a
fiber during lung residence.

Researchers have only recently begun to
scrutinize the mechanisms of fiber biological persistence and
biotransformation and their roles in lung injury. In the past, the
simple model offered was that fibers that enter the lung and
rapidly dissolve are innocuous, those that do not rapidly dissolve
are pathogenic.

Now, the situation appears more complex
that this according to recent experimental studies on E, 475, 901
glass fibers and rock wool fibers.

B-In Vitro Studies :

In vitro studies have demonstrated
widely varying dissolution rates for different fiber compositions.
These studies have identified two different types of dissolution
:

Fibers can dissolve congruently (i.e.,
all components dissolve at the same rate) or noncongruently (i.e.,
certain components dissolve more rapidly than others, leaving a
depleted fiber residuum; also called leaching).

Whereas congruent dissolution can lead
to the total dissolution and disappearance of fine fibers,
noncongruent leaching can weaken the infrastructure of the fiber
and thereby trigger transverse fragmentation, resulting in short
fiber segments that are biologically less active and more readily
removed from the lungs by phagocytic cells.

Leaching-induced changes in fiber
chemistry could also have an impact on the biological reactivity
of the fiber surface.

So, fibers that undergo rapid
biotransformation may be less toxic and less likely to cause lung
tumours because their altered dimensions or chemistry enhances
their clearance and may also decrease their biological
reactivity.

Mechanisms of fiber-induced
pathogenicity :

A-Lung Deposition :

Size and shape determine whether a fiber
is respirable. These two factors plus specific gravity (density)
determine where in the lung the fiber will deposit. Aerodynamic
diameter is a term that combines all three of these
characteristics.

Fibers longer than 5 µm and less
than 1.5 µm in diameter have the greatest potential to reach
the target areas of the lung and pleura. Fibers longer than 20
µm may be too long to be removed from the lung by alveolar
macrophages.

Altough fiber aerodynamic diameter
controls the entry and final site of deposition in the lung, fiber
durability is the critical basis for the accumulation of a lung
burder of fibers.

Other factors that may affect the
intrapulmonary fate of fibers are their rigidity, their surface
properties, and the architecture of their ends (smooth,
spicule-shaped edges, ect.)

B-Inflammatory Response :

The initial response to deposition of
foreign agents, including fibers, into the bronchio-alveolar
region is inflammation (alveolitis), which is initiated by lung
macrophages (one of the functions performed by this type of cell
is phagocytosis or " ingestion " of particulate matter ).

Activated macrophages migrate to the
site of fiber deposition and phagocytize (ingest) the fibers.
Individual macrophages appear to engulf short fibers completely,
but many macrophages may fuse as they engulf longer fibers. The
very long fibers may frustrate complete ingestion, resulting in
the release of a variety of cell messengers, reactive oxygen
species, and proteases from the cell macrophages.

The cell messengers signal the influx
and activation of more macrophages and other inflammatory
cells.

C-Fibrosis :

Biologically destructive agents that are
released from lung cells during inflammation attack the lung
walls, resulting in tissue necrosis. Tissue injury stimulates
tissue repair processes, including cell proliferation and
deposition of collagen by fibroblasts within the lung wall. During
prolonged tissue repair processes, normal lung morphology is
destroyed and replaced by scar tissue that is characterized by an
accumulation of collagen in the lung interstitium. This lung
scarring is called " lung fibrosis ".

Fibrotic scarring can also occur in the
mesothelial membranes (pleura) that enclose the lungs and line the
thoracic cavity.

Fibrotic lesions in the lung and
surrounding membranes reduce the efficiency of gas exchange,
leaving the individual with an excess of carbon dioxide and a
deficit of oxygen.

D-Neoplastic Tissue Response
:

Very recently, rodent inhalation studies
demonstrated for the first time that chronic inhalation of some
durable SVF types (RCF, E glass microfibers) at a dose 300-fold
greater than typical worker exposure could also be associated with
fibrosis and thoracic cancers.

475 durable glass as microfibers may
also induce mesothelioma in hamsters at the same level of
exposure.

E-Lung Cancer :

Lung cancer could develop as a
by-product of the chronic fibrosis that results from the chronic
lung irritation and caused by durable lung fibers. This mechanism
would require that the fiber be very biologically persistent in
the lung. Tobacco smoke is suggested to be a crucial factor in the
development of fiber-related cancers.

A second possible mechanism is that
inorganic fibers may act by direct genotoxic action to induce
neoplams.

F-Mesothelioma :

Malignant mesothelioma is cancer of the
mesothelial membranes, which cover the internal organs and line
the inner surfaces of the abdominal and thoracic cavities.

After chronic inhalation of high
concentrations of RCF, 42% of hamsters but only 1-3% of rats
developed thoracic mesotheliomas. As with lung cancer, the
mechanisms of fiber induction of mesothelioma are not well
understood.

After inhalation and deposition of
fibers, the next step in the development of fiber-associated
mesothelioma may be the translocation of fibers through the lung
wall into the pleural membranes.

Subsequent steps may involve the
development and advancement of pleural fibrosis in the same way
that lung fibrosis is theorized to be a mechanism in the
development of lung cancer.

As with lung cancer mechanisms, a second
potential mechanism of mesothelioma development would be direct
genotoxicity of the fibers in the pleural space.

G-Summary of Mechanisms :

Altough not completely understood, the
mechanisms of fiber-induced biological effects are believed to
include the following :

1-Inhaled fibers enter the deep
lung.

2-Fibers resist lung clearance and
degradation mechanisms.

3-Fibers are translocated into the lung
interstitium and, possibly, also the pleural membranes.

4-Fibers stimulate the cellular release
of inflammatory mediators.

5-The mediators initiate fibrosis and
epithelial cell proliferation.

In addition, fibers may also induce
neoplastic changes directly in the genetic material of the
cell.

Also affecting the potential
pathogenesis are other factors that compromise pulmonary health,
including previous or current disease or exposure to toxic
cofactors such as cigarette smoke, other dusts, or industrial
fumes.

It is important to note that lung
defense mechanisms can be overwhelmed by extreme experimental
exposure concentrations, resulting in lung injury that is not
specific to the particle type.

So, at overload concentrations, lung
injuries can be induced by innocuous dusts that, at normal
exposure levels, would be cleared from the lung before they are
able to accumulate sufficiently to inflict injury.

Each year, industrial hygienists analyze
more than 1,000 occupational exposure samples in at least 20 SVF
manufacturing plants in North America and Europe. Air samples are
also taken during insulation installation and in buildings where
SVF insulation and air filtration products are in use.

NIOSH method 7400 and the WHO reference
method established procedures for microscopically determining the
number of respirable fibers per cubic centimeter of air.

B-Synthetic Vitreous Fiber Exposure
Levels :

In general, exposure to SVFs during
manufacture, installation and final use has been very low or
undetectable.

In SVF manufacturing workplaces,
airborne fiber exposures have typically been less than 0.2 fiber
per cubic centimeter, with total particulate matter less than 1.0
mg/m3.

During installation of fiberglass, fiber
exposures averaged less than 0.5 fiber per cubic centimeter, with
a range of 0 to 20 fibers per cubic centimeter, and total
particulate matter averaged less than 4.2 mg/m3, with a range of
0.04 to 114.00 mg/m3.

Air samples were analyzed from a number
of public buildings in which fiberglass air filters were in use or
in which fiberglass insulation had been installed; these analyses
demonstrated no significant fiberglass exposure to the building
occupants.

Airborne concentrations of dust and
fibers in U.S. mineral wool plants are generally higher than in
U.S.glass wool facilities. Exposures during application or
installation are also typical higher for mineral wool products
than for similar glass wool products.

Industrial hygiene monitoring data
obtained on a regular basis at locations where RCF products are
manufactured show that exposures are generally less than 1.0 fiber
per cubic centimeter and often lower than 0.2 fiber per cubic
centimeter. During installation of RCF products, exposures can be
1 to 5 fibers per cubic centimeter or higher if appropriate
engineering controls and work practices are not followed.

C-Occupational Exposure Limits
:

A fiber may be defined as a lenghty
particle whose lenght/diameter ratio is equal or larger than 3. In
order to reach the lung alveolar region in man, a fiber must have
an aerodynamic diameter of less than 10 µm.

When conducting occupational exposure
studies to man-made fibers, only fibers considered hazardous to
workers due to their granulometric properties, are considered :

1-Lenght greater than 5 µm

2-Diameter less than 3 µm

3-Lenght/diameter ratio >3

QUEBEC'S EXPOSURE LIMITS

Substance

VEMP

Notes

1-Insulating wool fiber, slag wool

1
fiber/cm³

C2, EM

2-Insulating wool fiber, rock wool

1
fiber/cm³

C2, EM

3-Insulating wool fiber, glass
wool

2
fibers/cm³

C3

4-Fiberglass, continuous filaments

10mg/m³

Total dust

5-Refractory fibers, (ceramic or
others)

1
fiber/cm³

C3

6-Glass
microfibers

1
fiber/cm³

-

C2 : suspected carcinogen to
humans

C3 : confirmed carcinogen to
animals

EM : substance that should be kept at
the lowest practicable level

D-Occupational Exposure to Other
Compounds :

To accurately assess the toxicologic
potential of a substance in the workplace, all other substances
present in the environment must be considered.

Many chemicals may be present in
man-made fibers, which is not the case for asbestos fibers.

During the fabrication process many
chemicals may be added and account for up to 25 % of the weight of
these fibers, which may also be termed inorganic non-metallic
artificial fibers.

The chemicals added may be:

1-Antistatic agents

2-Antifungic agents

3-Hydrophobic agents

4-Anti-dust agents( mineral oils,
polypropylene glycol)

or

5-Binders(urea-formaldehyde and epoxy
resins, bitumen)

The presence of these additives may make
research on the toxicology of these fibers more
complicated.

The potential cumulative effects of
exposure to all these materials must be considered in any
operation to develop a sound plan for employee and environmental
health and safety.

Evaluation of synthetic
vitreous fibers :

A-International Agency for Research
on Cancer Evaluation (IARC) :

In 1971, the International Agency for
Reserach on Cancer (IARC) iniated a program by which to evaluate
data regarding the carcinogenic risk of chemicals to
humans.

In 1987, the IARC appointed a working
group of 20 scientists to evaluate the carcinogenic risk of
exposure to SVFs.

IARC classification :

*Group 1 : sufficient evidence of human
carcinogenicity

*Group 2A : probably carcinogenic to
humans

*Group 2B : possibly carcinogenic to
humans

*Group 3 : not classifiable as to human
carcinogenicity

*Group 4 : probably not carcinogenic to
humans

Glass wool was designated in group
2B.

Continuous filament (glass textile) was
designated as group 3.

Rock wool was clasified as group
2B.

Slag wool was classified as group
2B.

RCF(refractory ceramic fiber) was
designated as a group 2B substance.

B-International Program on Chemical
Safety Evaluation (IPCS) :

The IPCS is a joint venture of the
United Nations Environment Program, the International Labor
Organization and the WHO.

1-Possibility of transient effects of
skin and upper respiratory irritation.

2-Considering all results of animal
studies, that an increased risk of lung cancer in some sectors of
the SVF industry is biologically plausible.

They also recommended protective
equipment to guard against a potential elevation in lung cancer
risk for workers engaged in activities in which elevated airborne
exposure levels are possible.

For SVF in general the IPCS stated, "
The overall picture indicates that the possible risk of lung
cancer among the general public is very low, if there is any at
all, and should not be a cause for concern if the current low
levels of exposure continue ".

Occupational Health
Considerations :

A-Prevention and Protection
:

Whenever there exists a potential for
employees to be exposed to substsnces that either are known to be
harmful or have not been completely evaluated, the first step is
to mini-mize exposure to the lowest practicable level.

In SVF occupational settings,
modifications to the product design can sometimes reduce the
amount of dust that it releases during manufacture or
installation.

Exhaust ventilation can remove dusts at
their points of origin.

Appropriate work practices can also
limit the amount of dust generated; for exemple, vacuum cleaning
is better than dry-sweeping with a broom or with compressed
air.

SVF workers can further protect
themselves by wearing safety glasses or goggles to prevent eye
exposure, long-sleeved shirts and long pants to minimize skin
exposure, and respiratory protection to minimize dust
inhalation.

A careful evaluation of the workplace
should be conducted to determine the appropriate devices to be
used in an individual situation.

B-Monitoring Exposure and Health
:

Whenever employees are exposed to
potentially harmful substances, a program should be established to
monitor their exposure levels and health routinely.

First, exposure ranges and averages
should be determined for each operation or task.

Next, the appropriate type of personal
protective equipment should be determined for each task.

A medical surveillance program should be
established, including a review of general health, occupational
history, physical examination, clinical chemistries and blood
count, pulmonary function testing, a baseline chest radiograph,
and other testing as indicated by the occupational history.

For SVF workers, the focus should be on
respiratory and dermatologic health.

Exposure and health monitoring should
continue on a regular basis (e.g., yearly) or whenever processes
or products change. Findings should be reviewed regularly, both
for individuals and groups.

Health effects summary
:

The ban on the use of asbestos resulted
in a larger and larger use of substitution materials in many
industrial processes and in particular the use of man-made
vitreous fibers (MMVF).

In rodents, inhalational studies show
that glass insulation wools and slag wool produced no permanent
injury, even after 2 years of exposure to high concentrations (at
least 300-fold the concentrations to which human SVF workers
typically are exposed). In recent rodent inhalational studies, two
durable SVFcompositions were associated with permanent lung injury
: rock wool (MMVF21) induced fibrosis late in the study, and RCF
induced fibrosis and tumorigenesis. Other durable fibers are
pathogenic to animals : glass microfiber E may also induce
fibrosis and tumorigenesis in rats, fiber glass 475 induces
fibrosis and mesothelioma in hamsters but not in rats.

In man, the main part of known health
effects comes from data collected among workers of industries
producing these fibers, where the levels of exposure were low,
much lower than those encountered in many professional situations
by the finished product users.

Even if the relationship to the exposure
to rockwool fibers/slag wool fibers is not clearly established,
the observation of an excess of bronchopulmonary cancers among
workmen producing these fibres must prompt us to be vigilant and
to control levels of exposure to these fibers in the work
environment. The SMRs for bronchopulmonary cancer are lower among
workmen of glass wool production than among workmen of
rockwool/slag wool production.

Taking into account data observed in
experiments (excess of tumours) and preliminary information
obtained from man (suspicion of an excess of benign pleural
pathologies, and of respiratory functional impairment of the
obstructive type), an attitude even more careful is essential with
respect to refractory ceramic fibres.

These fibres were classified in category
2 (similar substances to cancerogenic substances for man) by the
European Communities.

Nothing currently makes it possible to
affirm that a risk of nonmalignant respiratory pathology exists
for man with rock, glass, and slag fibers. Nevertheless,
experimental data showed a real pathogenic effect for levels of
exposure close to those producing the same effects with asbestos.
Certain fibers, as some made from glass, appear sufficiently
soluble to have no irreversible effects.Others like ceramic fibers
are more suspicious.

The absence of sufficient experience
must prompt us to pursue epidemiologic and experimental studies,
and to introduce an effective prevention policy.